The WebMail extension (yep, they're not just for Firefox) lets you read both Hotmail and Yahoo! Mail in Mozilla Thunderbird. Thunderbird extension setup is a little different than Firefox: first, save the .xpi files to your computer. From the View menu -> Extensions dialog, install the files, then restart T-bird. The WebMail extension requires at least 2 XPI files, and some option setup, but it works like a charm.